Castlevania: Circle of the Moon, known in Japan as Akumajō Dracula: Circle of the Moon and as Castlevania in Europe, was released for the NintendoGame Boy Advance in 2001. The game was the first Castlevania game for the system. It was the last game in the franchise to bear the "Akumajō Dracula" title in Japan until Castlevania: Dawn of Sorrow was released. Subsequent entries adopted the "Castlevania" moniker.

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The year is 1830. At an old Austrian castle, the worshipers of Chaos attempted to revive the Lord of Darkness, Count Dracula. Vampire Hunter Morris Baldwin and his apprentices, Nathan Graves and Hugh Baldwin, heard of this and headed to Castlevania to stop Count Dracula's resurrection and restoration of power. However, they arrived a few seconds too late and Camilla, a minion of Count Dracula, started the rite to revive her master.

In order for Dracula to regain his former power, he would need to wait until the full moon ritual was completed. Before the group could fight Dracula and reseal him, Dracula destroyed the floor under Nathan and Hugh, causing them to plummet down a long shaft, leaving Morris alone with Dracula and Camilla. Fortunately, Nathan and Hugh survived the fall. Hugh ran off to save his father after telling Nathan to leave the castle. Nathan decided not to, as he too wanted to ensure that his master was safe. He then explored the large castle, fighting Dracula's minions along the way.

After fighting Camilla, Nathan learned that Hugh was possessed by Dracula after the vampire exploited his desire to become more powerful than Nathan. He wanted to prove that he was superior to Nathan so Morris would bestow the Hunter Whip to him instead of Nathan. When Nathan defeated him, Hugh came to his senses and leaves to find Morris.

Nathan arrived at the ceremonial room to vanquish Dracula and save his teacher from being sacrificed in the full moon ritual. While he fought Dracula, Hugh freed Morris and together they escaped the castle, leaving Nathan to finish vanquishing Dracula. He succeeds in beating Dracula's ultimate form. Before he was sealed, Dracula stated "It is a wasted effort. I will never be truly gone. So long as there is darkness and desire in the soul of man, I will continue to be revived". Nathan replied, "And we will be there to stop you. You can bet on that". Dracula dies and the castle crumbles. Nathan then leaves to continue his training with Morris and Hugh.

Note: The storyline from Castlevania: Circle of the Moon had been retroactively removed from the official Castlevania timeline by Koji Igarashi. However, many claim it has been reinstated after the release of Portrait of Ruin because it can be found in the timeline that accompanied the pre-order bundle for that game in America.

The gameplay in Circle of the Moon is similar to that of other post-Symphony of the NightCastlevania titles, in which the player must explore a gigantic castle filled with undead enemies and evil aberrations in a non-linear fashion. Generic enemies defeated by the player drop useful items and equipment, and also provide Experience points which increase the player character's power. Boss enemies provide plot progression, and their defeat allows the player to acquire new abilities such as a double-jump with which to progress further in the game, the only downside in boss battles is that Magic Crystals do not appear to replenish the player's HP and MP after the fight.

The game is based around the classic Castlevania action style. The main character, Nathan, uses a whip to attack the enemies, alongside the classic side weapons: Axe, Cross, Holy Water, Knife and Stopwatch. One of these sub-weapons may be carried at any time by the player, draining a supply of "Hearts" when used. In addition, the player may boost their character's statistics through equipping items such as armor. These statistics are Strength, which determines physical attack power, Defense, which determines resilience to damage, Intelligence, which determines potency of magic and the Magic Point (MP) recovery speed, and Luck, which modifies the frequency with which items are dropped by enemies.

Unique to this game is its DSS (Dual Set-Up System) for magic, based around magic cards. This gameplay mechanic allows the player to unleash dozens of interesting abilities via combinations of two different magic cards: one Action card and one Attribute card. For example, the player can combine the "Mercury Card" (Action card with the power of whip enhancement) with the "Salamander Card" (Attribute card with the power of fire), obtaining a fire elemental whip attack as a result. Each of these cards is dropped by certain enemies, with a probability based on the player character's "luck" statistic, thus adding another search element. Use of DSS cards drains the player's supply of MP.

Compared to the other Metroidvania-styled games with RPG mechanics, Circle of the Moon does not feature a shop. Therefore, the player cannot sell any of the acquired equipment, nor be able to accumulate gold.

This is also the only game wherein the player's base statistics (STR, INT, LCK) are set to three digits (100 each) from the start.

Magician Mode: Once the game has been completed, the player will receive a code that can be entered as their name when starting a new game: FIREBALL. This code will start the game in Magician Mode, where the player starts with all 20 DSS cards, and high magical ability, at the expense of reduced strength, defense and health.

Fighter Mode: By completing Magician Mode, another code is received: GRADIUS, which lets the player start the game in Fighter Mode. In this mode, no DSS cards can be obtained or used, but physical power is drastically increased.

Shooter Mode: In turn, completing Fighter Mode unlocks the code for Shooter Mode: CROSSBOW. This mode places an emphasis on combat using the sub-weapons, giving the player a very high maximum of Hearts to use them with (as well as halving the amount of hearts each sub-weapon uses), but giving a penalty to strength, defense and HP. Shooter Mode also has an unique sub-weapon, the Homing Dagger, which is only usable in this mode.

Thief Mode: Completing Shooter Mode unlocks the code for Thief Mode: DAGGER. This last alternate mode reduces all of the player's stats except Luck, which is boosted by an enormous amount. The idea in Thief Mode is to survive on whatever items and equipment the enemies drop, with greater Luck increasing the odds of getting items from each kill.

No alternate mode codes will function until the prior mode is actually completed.

The Battle Arena is an optional bonus area in the castle, consisting of 17 rooms. Each room contains a selection of standard enemies which must be defeated for the player to exit, however the Dual Set-Up System (DSS) is unavailable, with the player's MP dropping to zero as soon as they enter the Arena (though it is possible to use magic in small bursts by constantly using MP recovery items). Each room contains a tougher selection of enemies than the last. While completing the Arena as a whole is a stiff challenge, the player can leave the Arena between certain rooms, making the area ideal for gaining Experience.

When the player is about to enter to the Battle Arena, the following text is displayed:

To step beyond will be to test your ability in the battle arena.Your Mind Power will be drained. You will not be able to use DSS.Once you enter, you will not be able to leave until all are defeated.Though, you may be rewarded for you efforts. [sic]

Completing the entire battle arena grants the player 291,582 experience points in total from enemies defeated, and the "Shinning armor" [sic], an armor plate that significantly increases the player's Defense, with minor upgrades to Strength, Intelligence, and Luck.

There are also two DSS cards which can only be obtained from enemies in the arena.

A main sequence break is available in Circle of the Moon. After beating Adramelech, the "Goat Head" Boss, it is possible to go straight to the Underground Waterway, even before the water has been cleansed. On Magician mode, this can be accomplished with assistance from Pluto+Unicorn, and on other modes you can use Jupiter+Thunderbird to reduce water damage. On regular mode, it is possible to survive the Underground Waterway through judicious use of dash and double-jump, spending as little time in the water as possible and dodging every enemy. Some players have unknowingly done this, assuming that the Waterway was just a particularly difficult part of the already-hard game.

The DSS system contains a glitch. When Nathan activates his cards, pause and go back to the DSS screen. Then simply switch to any card you want, you will be able to cast magical effects from cards not even in your possession.

While there are no extra characters to play as, it is possible to change into a cartoon bear in Circle of the Moon. The bear, known as BearTank, comes from a Nintendo 64 Konami fighting game called "Rakuga Kids". To transform into BearTank, you will need a Bear Ring, dropped by the Skeleton Medalist enemy. The Skeleton Medalist is found in a secret room found by breaking a wall on the left side of the shaft Nathan falls down at the beginning of the game, and then another wall on the ceiling of that room. To transform into BearTank, equip a Bear Ring while using the Pluto + Black Dog spell. Unfortunately, the BearTank Morph still has all the vulnerabilities of the Skeleton Morph. It has a slightly different moveset from Nathan, however. While pressing B normally fires a missile in an arc toward the enemy, pressing UP+B fires a missile on a higher arc instead of a sub-weapon, and DOWN+B causes the bear to drop a bomb similar to Skeleton Bombers that causes double the damage of missile attacks, damage equal to Nathan's whip.[citation needed]